When I edit a file in photoshop it becomes completely broken. Random colours and shapes appear.

I did the following to produce this error:
1. Create a file with dimensions 4961 x 3508 pixels, 8 bit colors and adobe RGB (1998) color profile
2. Imported jpg picture
3. Imported png. Photoshop asked if I should convert colors and I pressed yes.
4. Hid the layer with the imported png and the photo looked completely crazy.

I have the Norwegian version of photoshop CC 2015 on a macbook retina with OSX yosemite 10.10.5

You're seeing video card driver bugs. Apple is aware of the well known video driver bugs in MacOS 10.10 and says they are working on a solution.
In the meantime, you can try disabling GPU drawing in Photoshop's preferences, and restarting Photoshop.